Low dielectric glass and fiber glass for electronic applications

ABSTRACT

Glass compositions are provided that are useful in electronic applications, e.g., as reinforcements in printed circuit board substrates. Reduced dielectric constants are provided relative to E-glass, and fiber forming properties are provided that are more commercially practical than D-glass.

BACKGROUND OF THE INVENTION

This invention relates to glass compositions that are adapted forformation into fibers that can be employed for reinforcing compositesubstrates comprising a printed circuit board (“PCB”). Moreparticularly, the invention relates to glass fiber reinforcements thathave electrical properties that permit enhancing performance of a PCB.

“D_(k)” is the dielectric constant of a material, also known as“permittivity” and is a measure of the ability of a material to storeelectric energy. A material to be used as a capacitor desirably has arelatively high D_(k), whereas a material to be used as part of a PCBsubstrate desirably has a low D_(k), particularly for high speedcircuits. D_(k) is the ratio of the charge that would be stored (i.e.,the capacitance) of a given material between two metal plates to theamount of charge that would be stored by a void (air or vacuum) betweenthe same two metal plates. “D_(f)” or dissipation factor is the measureof the loss of power in a dielectric material. D_(f) is the ratio of theresistive loss component of the current to the capacitive component ofcurrent, and is equal to the tangent of the loss angle. For high speedcircuitry, it is desired that the D_(f) of materials comprising a PCBsubstrate be relatively low.

PCB's have commonly been reinforced with glass fibers of the “E-glass”family of compositions, which is based on “Standard Specification forGlass Fiber Strands” D 578 American Society for Testing and Materials.By this definition, E-glass for electronic applications contains 5 to 10weight percent B₂O₃, which reflects recognition of the desirable effectof B₂O₃ on dielectric properties of glass compositions. E-glass fibersfor electronic applications typically have D_(k) in the range 6.7-7.3 at1 MHz frequency. Standard electronic E-glass is also formulated toprovide melting and forming temperatures conducive to practicalmanufacturing. Forming temperatures (the temperature at which theviscosity is 1000 poise), also referred to herein as T_(F), forcommercial electronic E-glass are typically in the range of 1170°C.-1250° C.

High performance printed circuit boards require substrate reinforcementshaving lower D_(k) compared to E-glass for better performance, i.e.,less noise signal transmission, for applications in telecommunicationand electronic computing. Optionally, reducing D_(f) relative to E-glassis also desired by the electronic industry. While the PCB industry has aneed for low dielectric fiber glass, manufacture of glass fiberreinforcement requires economical viability issues to be addressed inorder for low dielectric fibers to achieve successful commercialization.To this end, some low D_(k) glass compositions proposed in the prior artdo not adequately address the economic issues.

Some low dielectric glasses in the prior art are characterized by highSiO₂ content or high B₂O₃ content, or a combination of both high SiO₂and high B₂O₃. An example of the latter is known as “D-glass.” Detailedinformation on this approach to low D_(k) glass can be found in anarticle by L. Navias and R. L. Green, “Dielectric Properties of Glassesat Ultra-High Frequencies and their Relation to Composition,” J. Am.Ceram. Soc., 29, 267-276 (1946), in U.S. Patent Application 2003/0054936A1 (S. Tamura), and in patent application JP 3409806 B2 (Y. Hirokazu).Fibers of SiO₂ and glasses of the D-glass type have been used asreinforcement in fabric form for PCB substrates, e.g., laminatescomprised of woven fibers and epoxy resin. Although both of thoseapproaches successfully provide low D_(k), sometimes as low as about 3.8or 4.3, the high melting and forming temperatures of such compositionsresult in undesirably high costs for such fibers. D-glass fiberstypically require forming temperatures in excess of 1400° C., and SiO₂fibers entail forming temperatures on the order of about 2000° C.Furthermore, D-glass is characterized by high B₂O₃ content, as much as20 weight percent or greater. Since B₂O₃ is one of the most costly rawmaterials required for manufacturing conventional electronic E-glass,the use of much greater amounts of B₂O₃ in D-glass significantlyincreases its cost compared to E-glass. Therefore, neither SiO₂ norD-glass fibers provide a practical solution for manufacturing highperformance PCB substrate materials on a large scale.

Other low dielectric fiber glasses based on high B₂O₃ concentrations(i.e., 11 to 25 weight percent) plus other relatively costly ingredientssuch as ZnO (up to 10 weight percent) and BaO (up to 10 weight percent)have been described in JP 3409806B2 (Hirokazu), with reported D_(k)values in the 4.8-5.6 range at 1 MHz. The inclusion of BaO in thesecompositions is problematic because of cost as well as environmentalreasons. In spite of the high concentrations of the costly B₂O₃ in thecompositions of this reference, the fiber forming temperatures disclosedare relatively high, e.g., 1355° C.-1429° C. Similarly, other lowdielectric glasses based on high B₂O₃ concentrations (i.e., 14-20 weightpercent) plus relatively costly TiO₂ (up to 5 weight percent) have beendescribed in U.S. Patent Application 2003/0054936 A1 (Tamura), withD_(k)=4.6-4.8 and dissipation factor D_(f)=0.0007-0.001 at 1 MHz. InJapanese Patent Application JP 02154843A (Hiroshi et al.) there aredisclosed boron-free low dielectric glasses with D_(k) in the range5.2-5.3 at 1 MHz. Although these boron-free glasses provide low D_(k)with presumably relatively low raw material cost, their disadvantage isthat fiber forming temperatures at 1000 poise melt viscosity are high,between 1376° C. and 1548° C. Additionally, these boron-free glasseshave very narrow forming windows (the difference between the formingtemperature and the liquidus temperature), typically 25° C. or lower (insome cases negative), whereas a window of about 55° C. or higher wouldcommonly be considered expedient in the commercial fiber glass industry.

To improve PCB performance while managing the increase in cost, it wouldbe advantageous to provide compositions for fiber glasses that offersignificant improvements of electrical properties (D_(k) and/or D_(f))relative to E-glass compositions, and at the same time provide practicalforming temperatures lower than the SiO₂ and D-glass types and the otherprior art approaches to low dielectric glass discussed above. Tosignificantly lower raw material costs, it would be desirable tomaintain B₂O₃ content less than that of D-glass, e.g., below 13 weightpercent or below 12 percent. It can also be advantageous in somesituations for the glass composition to fall within the ASTM definitionof electronic E-glass, and thus to require no more than 10 weightpercent B₂O₃. It would also be advantageous to manufacture low D_(k)glass fibers without requiring costly materials such as BaO or ZnO thatare unconventional in the fiber glass industry. In addition,commercially practical glass compositions desirably have tolerance toimpurities in raw materials, which also permits the use of less costlybatch materials.

Since an important function of glass fiber in PCB composites is toprovide mechanical strength, improvements in electrical properties wouldbest be achieved without significantly sacrificing glass fiber strength.Glass fiber strength can be expressed in terms of Young's modulus orpristine tensile strength. It would also be desirable if new lowdielectric fiber glass solutions would be used to make PCB withoutrequiring major changes in the resins used, or at least withoutrequiring substantially more costly resins, as would be required by somealternative approaches.

SUMMARY OF THE INVENTION

The fiberizable glass compositions of this invention provide improvedelectrical performance (i.e., low D_(k) and/or low D_(f)) relative tostandard E-glass, while providing temperature-viscosity relationshipsthat are more conducive to commercially practical fiber forming thanprior art low D_(k) glass proposals. Another optional aspect of theinvention is that at least some of the compositions can be madecommercially with relatively low raw material batch cost. In one aspectof the invention, glass compositions comprise the followingconstituents, which may be in the form of glass fibers:

SiO₂ 60–68 weight percent;  B₂O₃ 7–13 weight percent;  Al₂O₃ 9–15 weightpercent;  MgO 8–15 weight percent;  CaO 0–4 weight percent; Li₂O 0–2weight percent; Na₂O 0–1 weight percent; K₂O 0–1 weight percent; Fe₂O₃0–1 weight percent; F₂ 0–1 weight percent; TiO₂ 0–2 weight percent.

In some embodiments, the compositions of the invention are characterizedby relatively low content of CaO, for example on the order of about 0-4weight percent. In yet other embodiments, the CaO content can be on theorder of about 0-3 weight percent. In general, minimizing the CaOcontent yields improvements in electrical properties, and the CaOcontent has been reduced to such low levels in some embodiments that itcan be considered an optional constituent. On the other hand, the MgOcontent is relatively high for glasses of this type, wherein in someembodiments the MgO content is double that of the CaO content (on aweight percent basis). Some embodiments of the invention can have MgOcontent greater than about 6.0 weight percent, and in other embodimentsthe MgO content can be greater than 7.0 weight percent.

As noted above, some low D_(k) compositions of the prior art have thedisadvantage of requiring the inclusion of substantial amounts of BaO,and it can be noted that BaO is not required in the glass compositionsof the present invention. Although the advantageous electrical andmanufacturing properties of the invention do not preclude the presenceof BaO, the absence of deliberate inclusions of BaO can be considered anadditional advantage of some embodiments of the present invention. Thus,embodiments of the present invention can be characterized by thepresence of less than 1.0 weight percent BaO. In those embodiments inwhich only trace impurity amounts are present, the BaO content can becharacterized as being no more than 0.05 weight percent.

The compositions of the invention include B₂O₃ in amounts less that theprior art approaches that rely upon high B₂O₃ to achieve low D_(k). Thisresults in significant cost savings. In some embodiments the B₂O₃content need be no more than 13 weight percent, or no more than 12weight percent. Some embodiments of the invention also fall within theASTM definition of electronic E-glass, i.e., no more than 10 weightpercent B₂O₃.

In the composition set forth above, the constituents are proportioned soas to yield a glass having a dielectric constant lower than that ofstandard E-glass. With reference to a standard electronic E-glass forcomparison, this may be less than about 6.7 at 1 MHz frequency. In otherembodiments, the dielectric constant (D_(k)) may be less than 6 at 1 MHzfrequency. In other embodiments, the dielectric constant (D_(k)) may beless than 5.8 at 1 MHz frequency. Further embodiments exhibit dielectricconstants (D_(k)) less than 5.6 or even lower at 1 MHz frequency.

The compositions set forth above possess desirable temperature-viscosityrelationships conducive to practical commercial manufacture of glassfibers. In general, lower temperatures are required for making fiberscompared to the D-glass type of composition in the prior art. Thedesirable characteristics may be expressed in a number of ways, and theymay be attained by the compositions of the present invention singly orin combination. In general, glass compositions within the ranges setforth above can be made that exhibit forming temperatures (T_(F)) at1000 poise viscosity no greater than 1370° C. The T_(F) of someembodiments are no greater than 1320° C., or no greater than 1300° C.,or no greater than 1290° C. These compositions also encompass glasses inwhich the difference between the forming temperature and the liquidustemperature (T_(L)) is positive, and in some embodiments the formingtemperature is at least 55° C. greater than the liquidus temperature,which is advantageous for commercial manufacturing of fibers from theseglass compositions.

In general, minimizing alkali oxide content of the glass compositionsassists lowering D_(k). In those embodiments in which it is desired tooptimize reduction of D_(k) the total alkali oxide content is no morethan 2 weight percent of the glass composition. In compositions of thepresent invention it has been found that minimizing Na₂O and K₂O aremore effective in this regard than Li₂O. The presence of alkali oxidesgenerally results in lower forming temperatures. Therefore, in thoseembodiments of the invention in which providing relatively low formingtemperatures is a priority, Li₂O is included in significant amounts,e.g. at least 0.4 weight percent. For this purpose, in some embodimentsthe Li₂O content is greater than either the Na₂O or K₂O contents, and inother embodiments the Li₂O content is greater than the sum of the Na₂Oand K₂O contents, in some embodiments greater by a factor of two ormore.

In addition to or instead of the features of the invention describedabove, the compositions of the present invention can be utilized toprovide glasses having dissipation factors (D_(f)) lower than standardelectronic E-glass. In some embodiments D_(F) is no more than 0.0150 at1 GHz, and in other embodiments no more than 0.0100 at 1 GHz.

One advantageous aspect of the invention present in some of theembodiments is reliance upon constituents that are conventional in thefiber glass industry and avoidance of substantial amounts ofconstituents whose raw material sources are costly. For this aspect ofthe invention, constituents in addition to those explicitly set forth inthe compositional definition of the glasses of the present invention maybe included even though not required, but in total amounts no greaterthan 5 weight percent. These optional constituents include melting aids,fining aids, colorants, trace impurities and other additives known tothose of skill in glassmaking. Relative to some prior art low D_(k)glasses, no BaO is required in the compositions of the presentinvention, but inclusions of minor amounts of BaO (e.g., up to about 1weight percent) would not be precluded. Likewise, major amounts of ZnOare not required in the present invention, but in some embodiments minoramounts (e.g., up to about 2.0 weight percent) may be included. In thoseembodiments of the invention in which optional constituents areminimized, the total of optional constituents is no more than 2 weightpercent, or no more than 1 weight percent. Alternatively, someembodiments of the invention can be said to consist essentially of thenamed constituents.

DETAILED DESCRIPTION

To lower D_(k) and D_(f), including SiO₂ and B₂O₃, which have lowelectrical polarizability, is useful in the compositions of the presentinvention. Although B₂O₃ by itself can be melted at a low temperature(350° C.), it is not stable against moisture attack in ambient air andhence, a fiber of pure B₂O₃ is not practical for use in PCB laminates.Both SiO₂ and B₂O₃ are network formers, and the mixture of two wouldresult in significantly higher fiber forming temperature than E-glass,as is the case with D-glass. To lower fiber-forming temperature, MgO andAl₂O₃ are included, replacing some of the SiO₂. Calcium oxide (CaO) andSrO can be also used in combination with MgO, although they are lessdesirable than MgO because both have higher polarizability than MgO.

To lower batch cost, B₂O₃ is utilized at lower concentrations than inD-glass. However, sufficient B₂O₃ is included to prevent phaseseparation in glass melts, thereby providing better mechanicalproperties for glass fibers made from the compositions.

The choice of batch ingredients and their cost are significantlydependent upon their purity requirements. Typical commercialingredients, such as for E-glass making, contain impurities of Na₂O,K₂O, Fe₂O₃ or FeO, SrO, F₂, TiO₂, SO₃, etc. in various chemical forms. Amajority of the cations from these impurities would increase the D_(k)of the glasses by forming nonbridging oxygens with SiO₂ and/or B₂O₃ inthe glass.

Sulfate (expressed as SO₃) may also be present as a refining agent.Small amounts of impurities may also be present from raw materials orfrom contamination during the melting processes, such as SrO, BaO, Cl₂,P₂O₅, Cr₂O₃, or NiO (not limited to these particular chemical forms).Other refining agents and/or processing aids may also be present such asAs₂O₃, MnO, MnO₂, Sb₂O₃, or SnO₂, (not limited to these particularchemical forms). These impurities and refining agents, when present, areeach typically present in amounts less than 0.5% by weight of the totalglass composition. Optionally, elements from rare earth group of thePeriodic Table of the Elements may be added to compositions of thepresent invention, including atomic numbers 21 (Sc), 39 (Y), and 57 (La)through 71 (Lu). These may serve as either processing aids or to improvethe electrical, physical (thermal and optical), mechanical, and chemicalproperties of the glasses. The rare earth additives may be included withregard for the original chemical forms and oxidization states. Addingrare earth elements is considered optional, particularly in thoseembodiments of the present invention having the objective of minimizingraw material cost, because they would increase batch costs even at lowconcentrations. In any case, their costs would typically dictate thatthe rare earth components (measured as oxides), when included, bepresent in amounts no greater than about 0.1-1.0% by weight of the totalglass composition.

The invention will be illustrated through the following series ofspecific embodiments. However, it will be understood by one of skill inthe art that many other embodiments are contemplated by the principlesof the invention.

The glasses in these examples were made by melting mixtures of reagentgrade chemicals in powder form in 10% Rh/Pt crucibles at thetemperatures between 1500° C. and 1550° C. (2732° F.-2822° F.) for fourhours. Each batch was about 1200 grams. After the 4-hour melting period,the molten glass was poured onto a steel plate for quenching. Tocompensate volatility loss of B₂O₃ (typically about 5% in laboratorybatch melting condition for the 1200 gram batch size), the boronretention factor in the batch calculation was set at 95%. Other volatilespecies, such as fluoride and alkali oxides, were not adjusted in thebatches for their emission loss because of their low concentrations inthe glasses. The compositions in the examples represent as-batchedcompositions. Since reagent chemicals were used in preparing the glasseswith an adequate adjustment of B₂O₃, the as-batched compositionsillustrated in the invention are considered to be close to the measuredcompositions.

Melt viscosity as a function of temperature and liquidus temperaturewere determined by using ASTM Test Method C965 “Standard Practice forMeasuring Viscosity of Glass Above the Softening Point,” and C829“Standard Practices for Measurement of Liquidus Temperature of Glass bythe Gradient Furnace Method,” respectively.

A polished disk of each glass sample with 40 mm diameter and 1-1.5 mmthickness was used for electrical property and mechanical propertymeasurements, which were made from annealed glasses. Dielectric constant(D_(k)) and dissipation factor (D_(f)) of each glass were determinedfrom 1 MHz to 1 GHz by ASTM Test Method D150 “Standard Test Methods forA-C Loss Characteristics and Permittivity (Dielectric Constant) of SolidElectrical Insulating Materials.” According to the procedure, allsamples were preconditioned at 25° C. under 50% humidity for 40 hours.Selective tests were performed for glass density using ASTM Test MethodC729 “Standard Test Method for Density of Glass by the Sink-FloatComparator,” for which all samples were annealed.

For selected compositions, a microindentation method was used todetermine Young's modulus (from the initial slope of the curve ofindentation loading—indentation depth, in the indenter unloading cycle),and microhardness (from the maximum indentation load and the maximumindentation depth). For the tests, the same disk samples, which had beentested for D_(k) and D_(f), were used. Five indentation measurementswere made to obtain average Young's modulus and microhardness data. Themicroindentation apparatus was calibrated using a commercial standardreference glass block with a product name BK7. The reference glass hasYoung's modulus 90.1 GPa with one standard deviation of 0.26 GPa andmicrohardness 4.1 GPa with one standard deviation of 0.02 GPa, all ofwhich were based on five measurements.

All compositional values in the examples are expressing in weightpercent.

Table 1 Compositions

Examples 1-8 provide glass compositions (Table 1) by weight percentage:SiO₂ 62.5-67.5%, B₂O₃ 8.4-9.4%, Al₂O₃ 10.3-16.0%, MgO 6.5-11.1%, CaO1.5-5.2%, Li₂O 1.0%, Na₂O 0.0%, K₂O 0.8%, Fe₂O₃ 0.2-0.8%, F₂ 0.0%, TiO₂0.0%, and sulfate (expressed as SO₃) 0.0%.

The glasses were found to have D_(k) of 5.44-5.67 and D_(f) of0.0006-0.0031 at 1 MHz, and D_(k) of 5.47-6.67 and D_(f) of0.0048-0.0077 at 1 GHz frequency. The electric properties of thecompositions in Series III illustrate significantly lower (i.e.,improved) D_(k) and D_(f) over standard E-glass with D_(k) of 7.29 andD_(f) of 0.003 at 1 MHz and D_(k) of 7.14 and D_(f) of 0.0168 at 1 GHz.

In terms of fiber forming properties, the compositions in Table 1 haveforming temperatures (T_(F)) of 1300-1372° C. and forming windows(T_(F)-T_(L)) of 89-222° C. This can be compared to a standard E-glasswhich has T_(F) typically in the range 1170-1215° C. To prevent glassdevitrification in fiber forming, a forming window (T_(F)-T_(L)) greaterthan 55° C. is desirable. All of the compositions in Table 1 exhibitsatisfactory forming windows. Although the compositions of Table 1 havehigher forming temperatures than E-glass, they have significantly lowerforming temperatures than D-glass (typically about 1410° C.).

TABLE 1 EXAMPLE: 1 2 3 4 5 6 7 8 Al₂O₃ 11.02 9.45 11.64 12.71 15.9510.38 10.37 11.21 B₂O₃ 8.55 8.64 8.58 8.56 8.46 8.71 9.87 9.28 CaO 5.105.15 3.27 2.48 1.50 2.95 2.01 1.54 CoO 0.00 0.00 0.00 0.00 0.00 0.000.00 0.62 Fe₂O₃ 0.39 0.40 0.39 0.39 0.39 0.53 0.80 0.27 K₂O 0.77 0.780.77 0.77 0.76 0.79 0.79 0.78 Li₂O 0.98 0.99 0.98 0.98 0.97 1.00 1.001.00 MgO 6.70 7.44 8.04 8.69 9.24 10.39 11.05 11.04 SiO₂ 66.48 67.1666.32 65.42 62.72 65.26 64.12 64.26 Properties D_(k), 1 MHz 5.62 5.595.44 5.47 5.50 5.67 5.57 5.50 D_(k), 1 GHz 5.65 5.62 5.46 5.47 5.53 5.675.56 5.50 D_(f), 1 MHz 0.0010 0.0006 0.0016 0.0008 0.0020 0.0031 0.00120.0010 D_(f), 1 GHz 0.0048 0.0059 0.0055 0.0051 0.0077 0.0051 0.00530.0049 T_(L) (° C.) 1209 1228 1215 1180 1143 1219 1211 1213 T_(F) (° C.)1370 1353 1360 1372 1365 1319 1300 1316 T_(F) − T_(L) (° C.) 161 125 145192 222 100 89 103

Table 2 Compositions

Examples 9-15 provide glass compositions: SiO₂ 60.8-68.0%, B₂O₃ 8.6 and11.0%, Al₂O₃ 8.7-12.2%, MgO 9.5-12.5%, CaO 1.0-3.0%, Li₂O 0.5-1.5%, Na₂O0.5%, K₂O 0.8%, Fe₂O₃ 0.4%, F₂ 0.3%, TiO₂ 0.2%, and sulfate (expressedas SO₃) 0.0%.

The glasses were found to have D_(k) of 5.55-5.95 and D_(f) of0.0002-0.0013 at 1 MHz, and D_(k) of 5.54-5.94 and D_(f) of0.0040-0.0058 at 1 GHz frequency. The electric properties of thecompositions in Table 2 illustrate significantly lower (improved) D_(k)and D_(f) over standard E-glass with D_(k) of 7.29 and D_(f) of 0.003 at1 MHz, and D_(k) of 7.14 and D_(f) of 0.0168 at 1 GHz.

In terms of mechanical properties, the compositions of Table 2 haveYoung's modulus of 86.5-91.5 GPa and microhardness of 4.0-4.2 GPa, bothof which are equal or higher than standard E glass that has Young'smodulus of 85.9 GPa and microhardness of 3.8 GPa. The Young's moduli ofthe compositions in the Table 2 are also significantly higher thanD-glass which is about 55 GPa based on literature data.

In terms of fiber forming properties, the compositions of Table 2 haveforming temperature (T_(F)) of 1224-1365° C., and forming windows(T_(F)-T_(L)) of 6-105° C. as compared to standard E-glass having T_(F)in the range 1170-1215° C. Some, but not all, of the Table 2compositions have a forming window (T_(F)-T_(L)) greater than 55° C.,which is considered preferable in some circumstances to avoid glassdevitrification in commercial fiber forming operations. The Table 2compositions have lower forming temperatures than those of D-glass(1410° C.), although higher than E-glass.

TABLE 2 EXAMPLE: 9 10 11 12 13 14 15 Al₂O₃ 12.02 11.88 10.41 12.08 12.188.76 12.04 B₂O₃ 10.98 10.86 9.90 8.71 8.79 8.79 8.68 CaO 1.07 2.90 2.022.95 1.09 1.09 2.94 F₂ 0.32 0.31 0.32 0.32 0.32 0.32 0.32 Fe₂O₃ 0.400.39 0.40 0.40 0.40 0.40 0.40 K₂O 0.78 0.77 0.79 0.79 0.79 0.79 0.78Li₂O 0.50 0.49 1.00 0.50 1.51 1.51 1.49 MgO 12.35 9.56 11.10 12.41 12.519.81 9.69 Na₂O 0.51 0.51 0.52 0.52 0.52 0.52 0.52 SiO₂ 60.87 62.13 63.3561.14 61.68 67.80 62.95 TiO₂ 0.20 0.20 0.20 0.20 0.20 0.20 0.20Properties D_(k), 1 MHz 5.69 5.55 5.74 5.84 5.95 5.60 5.88 D_(k), 1 GHz5.65 5.54 5.71 5.83 5.94 5.55 5.86 D_(f), 1 MHz 0.0007 0.0013 0.00070.0006 0.0002 0.0002 0.0011 D_(f), 1 GHz 0.0042 0.0040 0.0058 0.00430.0048 0.0045 0.0053 T_(L) (° C.) 1214 1209 1232 1246 1248 1263 1215T_(F) (° C.) 1288 1314 1287 1277 1254 1365 1285 T_(F) − T_(L) (° C.) 74105 55 31 6 102 70 E (GPa) 90.5 87.4 86.8 86.5 89.6 87.2 91.5 H (GPa)4.12 4.02 4.02 4.03 4.14 4.07 4.19

TABLE 3 EXAMPLES: 16 17 18 19 20 Al₂O₃ 10.37 11.58 8.41 11.58 12.05 B₂O₃8.71 10.93 10.66 8.98 8.69 CaO 2.01 2.63 3.02 1.78 2.12 F₂ 0.32 0.300.30 0.30 0.30 Fe₂O₃ 0.40 0.27 0.27 0.27 0.27 K₂O 0.79 0.25 0.25 0.160.10 Li₂O 0.50 1.21 1.53 0.59 1.40 MgO 11.06 10.04 9.65 11.65 10.57 Na₂O0.52 0.25 0.57 0.35 0.15 SiO₂ 65.13 62.55 65.35 64.35 64.35 TiO₂ 0.200.00 0.00 0.00 0.00 Total 100.00 100.00 100.00 100.00 100.00 D_(k), 1MHz 5.43 5.57 5.30 5.42 D_(k), 1 GHz 5.33 5.48 5.22 5.33 D_(f), 1 MHz0.0057 0.0033 0.0031 0.0051 D_(f), 1 GHz 0.0003 0.0001 0.0008 0.0014T_(L) (° C.) 1231 1161 1196 1254 1193 T_(F) (° C.) 1327 1262 1254 13121299 T_(F) − T_(L) (° C.) 96 101 58 58 106 T_(M) (° C.) 1703 1592 16411634 1633 E (GPa) 85.3 86.1 85.7 91.8 89.5 Std E (GPa) 0.4 0.6 2.5 1.71.5 H (GPa) 3.99 4.00 4.03 4.22 4.13 Std H (GPa) 0.01 0.02 0.09 0.080.05 EXAMPLES: 21 22 23 24 25 26 Al₂O₃ 12.04 12.04 12.04 12.04 12.0412.54 B₂O₃ 8.65 8.69 10.73 10.73 11.07 8.73 CaO 2.06 2.98 2.98 2.98 2.982.88 F₂ 0.45 0.45 0.45 0.45 0.45 2.00 Fe₂O₃ 0.35 0.35 0.35 0.35 0.350.35 K₂O 0.4 0.4 0.4 0.4 0.4 0.40 Li₂O 1.53 1.05 1.05 0.59 0.48 MgO10.47 10.62 9.97 11.26 11.26 11.26 Na₂O 0.5 0.5 0.5 0.5 0.5 0.50 SiO₂63.05 62.42 61.03 60.2 59.97 61.34 TiO₂ 0.5 0.5 0.5 0.5 0.5 Total 100.00100.00 100.00 100.00 100.00 100.00 D_(k), 1 MHz 5.75 5.73 5.61 5.64 5.635.35 D_(k), 1 GHz 5.68 5.61 5.55 5.54 5.49 5.38 D_(f), 1 MHz 0.0040.0058 0.0020 0.0046 0.0040 0.0063 D_(f), 1 GHz 0.0021 0.0024 0.00340.0019 0.0023 0.0001 T_(L) (° C.) 1185 1191 1141 1171 1149 1227 T_(F) (°C.) 1256 1258 1244 1246 1249 1301 T_(F) − T_(L) (° C.) 71 67 103 75 100T_(M) (° C.) 1587 1581 1587 1548 1553 E (GPa) Std E (GPa) H (GPa) Std H(GPa) σ_(f) (KPSI/GPa) 475.7/ 520.9/ 466.5/ 522.0 3.28 3.59 3.22 Stdσ_(f) 37.3/ 18.3/ 41.8/ 18.70 (KPSI/GPa) 0.26 0.13 0.29 Density 2.4209*2.4324* 2.4348* (g/cm³)

TABLE 4 EXAMPLE: 27 28 E-Glass Al₂O₃ 12.42 12.57 13.98 B₂O₃ 9.59 8.595.91 CaO 0.11 0.10 22.95 F₂ 0.35 0.26 0.71 Fe₂O₃ 0.21 0.21 0.36 K₂O 0.180.18 0.11 Li₂O 0.80 1.01 0 MgO 10.25 10.41 0.74 Na₂O 0.15 0.18 0.89 SiO₂65.47 65.96 54.15 TiO₂ 0.17 0.17 0.07 D_(k), 1 MHz 5.3 5.4 7.3 D_(k), 1GHz 5.3 5.4 7.1 D_(f), 1 MHz 0.003 0.008 D_(f), 1 GHz 0.011 0.012 0.0168T_(L) (° C.) 1184 1201 1079 T_(F) (° C.) 1269 1282 1173 T_(F) − T_(L) (°C.) 85 81 94 E (GPa) H (GPa) 3.195 3.694

1. A glass composition suitable for fiber forming comprising: SiO₂ 60–68weight percent; B₂O₃ 7–13 weight percent; Al₂O₃ 9–15 weight percent; MgO8–15 weight percent; CaO 0–4 weight percent; Li₂O 0–2 weight percent;Na₂O 0–1 weight percent; K₂O 0–1 weight percent; Fe₂O₃ 0–1 weightpercent; F₂ 0–1 weight percent; TiO₂ 0–2 weight percent; and otherconstituents 0–5 weight percent total.


2. The composition of claim 1 wherein the CaO content is 0-3 weightpercent.
 3. The composition of claim 1 wherein the CaO content is 0-2weight percent.
 4. The composition of claim 1 wherein the CaO content is0-1 weight percent.
 5. The composition of claim 1 wherein the MgOcontent is 8-13 weight percent.
 6. The composition of claim 1 whereinthe MgO content is 9-12 weight percent.
 7. The composition of claim 1wherein the TiO₂ content is 0-1 weight percent.
 8. The composition ofclaim 1 wherein the B₂O₃ content is 7-12 weight percent.
 9. Thecomposition of claim 1 wherein the B₂O₃ content is no more than 10weight percent.
 10. The composition of claim 1 wherein the Al₂O₃ contentis 9-14 weight percent.
 11. The composition of claim 1 wherein the Al₂O₃content is 10-13 weight percent.
 12. The composition of claim 1 whereinthe constituents are selected to provide a glass having a dielectricconstant (D_(k)) less than 6.7 at 1 MHz frequency.
 13. The compositionof claim 1 wherein the constituents are selected to provide a glasshaving a dielectric constant (D_(k)) less than 6 at 1 MHz frequency. 14.The composition of claim 1 wherein the constituents are selected toprovide a glass having a dielectric constant (D_(k)) less than 5.8 at 1MHz frequency.
 15. The composition of claim 1 wherein the constituentsare selected to provide a glass having a dielectric constant (D_(k))less than 5.6 at 1 MHz frequency.
 16. The composition of claim 1 whereinthe constituents are selected to provide a forming temperature T_(F) at1000 poise viscosity no greater than 1370° C.
 17. The composition ofclaim 1 wherein the constituents are selected to provide a formingtemperature T_(F) at 1000 poise viscosity no greater than 1320° C. 18.The composition of claim 1 wherein the constituents are selected toprovide a forming temperature T_(F) at 1000 poise viscosity no greaterthan 1300° C.
 19. The composition of claim 1 wherein the constituentsare selected to provide a forming temperature T_(F) at 1000 poiseviscosity no greater than 1290° C.
 20. The composition of claim 16wherein the constituents are selected to provide a liquidus temperature(T_(L)) at least 55° C. below the forming temperature.
 21. Thecomposition of claim 17 wherein the constituents are selected to providea liquidus temperature (T_(L)) at least 55° C. below the formingtemperature.
 22. The composition of claim 18 wherein the constituentsare selected to provide a liquidus temperature (T_(L)) at least 55° C.below the forming temperature.
 23. The composition of claim 19 whereinthe constituents are selected to provide a liquidus temperature (T_(L))at least 55° C. below the forming temperature.
 24. The composition ofclaim 1 wherein the Li₂O content is 0.4-2.0 weight percent.
 25. Thecomposition of claim 24 wherein the Li₂O content is greater than the(Na₂O+K₂O) content.
 26. The composition of claim 1 wherein the (Li₂ONa₂O,+K₂O) content is less than 2 weight percent.
 27. The composition ofclaim 1 wherein the composition contains 0 -1 weight percent of BaO and0-2 weight percent ZnO.
 28. The composition of claim 1 wherein thecomposition contains essentially no BaO and essentially no ZnO.
 29. Thecomposition of claim 1 wherein other constituents, if any, are presentin a total amount of 0-2 weight percent.
 30. The composition of claim 1wherein other constituents, if any, are present in a total amount of 0-1weight percent.
 31. A glass composition suitable for fiber formingcomprising: B₂O₃ less than 14 weight percent; Al₂O₃ 9–15 weight percent;MgO 8–15 weight percent; CaO 0–4 weight percent; SiO₂ 60–68 weightpercent;

wherein the glass exhibits a dielectric constant (D_(k)) less than 6.7and forming temperature (T_(F)) at 1000 poise viscosity no greater than1370° C.
 32. The composition of claim 31l wherein the CaO content is 0-1weight percent.
 33. A glass composition suitable for fiber formingcomprising: SiO₂ 60–68 weight percent  B₂O₃ 7–13 weight percent  Al₂O₃9–15 weight percent  MgO 8–15 weight percent  CaO 0–3 weight percentLi₂O 0.4–2 weight percent   Na₂O 0–1 weight percent K₂O 0–1 weightpercent Fe₂O₃ 0–1 weight percent F₂ 0–1 weight percent TiO₂ 0–2 weightpercent

wherein the glass exhibits a dielectric constant (D_(k)) less than 5.9and forming temperature (T_(F)) at 1000 poise viscosity no greater than1300° C.
 34. A glass composition suitable for fiber forming consistingessentially of: SiO₂ 60–68 weight percent;  B₂O₃ 7–11 weight percent; Al₂O₃ 9–13 weight percent;  MgO 8–13 weight percent;  CaO 0–3 weightpercent; Li₂O 0.4–2 weight percent;   Na₂O 0–1 weight percent; K₂O 0–1weight percent; (Na₂O + K₂O + Li₂O) 0–2 weight percent; Fe₂O₃ 0–1 weightpercent; F₂ 0–1 weight percent; TiO2 0–2 weight percent.


35. The composition of claim 34 wherein the CaO content is 0-1 weightpercent.
 36. The composition of claim 35 wherein the B₂O₃ content is nomore than 10 weight percent.